I know several who do carry two, including one who just graduated from the academy where they teach them to carry a backup.
Masaad Ayoob recomends carrying two on different sides in case you get shot on one arm and can't draw your weapon from that side.
I don't usually carry two guns either but to call it dumb that anyone else would is well, dumb.
A lot of cops carry a backup. Most don't. Some are probably required to. Some are probably required to not. Some make the choice, either way. If a cop feels he/she needs to carry a backup, I am all for it. A cop is likely to need to use his firearm on any given day. A citizen carrying concealed is not likely to ever need it. That's the difference.
I have tremendous respect for Mr. Ayoob. If he's recommending that your average CCW citizen carry two guns, then I think he's making a mistake. However, I don't think that's what he's recommending. I believe, when he makes those statements, he's making them for people in a high-risk environment or occupation. In those conditions, it might make sense. For the rest of us, it doesn't.
I can call any behavior dumb if I think it is. I'm not calling the person dumb, just the behavior.